Port forward from FusionHub to existing Firewall when drop in mode shares 1 public IP

Hi everyone, having issues getting something working so would love some feedback. In this config:

Existing Firewall at with a web server connected at
Internet has a /29, the Gateway is 72.X.X.113 and the Firewall is assigned 72.X.X.114

The existing Firewall port forwards TCP 80 to the, so when accessing 72.X.X.114 remotely, the web server comes up as expected.

I connect a HD2 using Drop in Mode and assign 72.X.X.115 to it which is connected to FusionHub via PepVPN. FusionHub is 77.X.X.10. On FusionHub, I turn on TCP 80 port forwarding over WAN1 to 72.X.X.114. When I do this, I can access 77.X.X.10 and I’m brought to the web server also which works.

Where I’m confused/stuck though is in a scenario where there’s only 1 single public IP vs a /29 how to do the same thing. I changed the config for testing purposes and instead of assigning the HD2 72.X.X.115 I told it to share the 72.X.X.114 IP of the Firewall, but I can no longer access the web server at via FusionHub’s public IP when I do this. Tried changing a bunch of settings and other port forward options but nothing seems to work. The IP assigned to the HD2 I left as the default which was I believe

So just trying to figure out what the correct process is when using drop in mode with 1 public IP when I want to port forward from FusionHub to the existing firewall, as it works great when there’s multiple public IPs, but most of our customers only have 1.